Help needed! urbackup_snapshot_helper test not working on BTRFS drive

I’m trying to get urbackup 2.4.9.0 to work on Ubuntu 19.10 x86.
I have a 2T usb drive partitioned as btrfs, but it’s not recognized as btrfs by urbackup_snapshot_helper.

/etc/urbackup/backupfolder contains:
/media/BACKUP/urbackup

The test program suggested in https://www.urbackup.org/administration_manual.html#x1-10600011.6.3 says that /media/BACKUP/urbackup/testA5… is not btrfs. I don’t know where the …TestA5… came from.

% urbackup_snapshot_helper test
Testing for btrfs…
ERROR: not a btrfs filesystem: /media/BACKUP/urbackup/testA54hj5luZtlorr494
TEST FAILED: Creating test btrfs subvolume failed
Testing for zfs…

parted -l /dev/sda1

Model: Seagate Ultra Slim MT (scsi)
Disk /dev/sda: 2000GB
Sector size (logical/physical): 512B/4096B
Partition Table: msdos
Disk Flags:

Number Start End Size Type File system Flags
1 1049kB 2000GB 2000GB primary btrfs boot
Here are the contents of the usb drive:

% ls -l /media/BACKUP/
/media/BACKUP/:
total 4
drwxr-xr-x 5 urbackup urbackup 4096 Oct 18 17:45 urbackup/

/media/BACKUP/urbackup:
total 12
drwxr-x— 2 urbackup urbackup 4096 Jan 21 2018 clients/
drwxr-x— 2 urbackup urbackup 4096 Oct 16 03:26 urbackup/
drwxr-x— 2 urbackup urbackup 4096 Oct 18 16:56 urbackup_tmp_files/

/media/BACKUP/urbackup/clients:
total 0

/media/BACKUP/urbackup/urbackup:
total 60620
-rwxr-x— 1 urbackup urbackup 52428800 Oct 16 03:25 backup_server.db*
-rwxr-x— 1 urbackup urbackup 9051672 Oct 16 03:25 backup_server.db-wal*
-rwxr-x— 1 urbackup urbackup 16384 Oct 16 03:26 backup_server_files.db*
-rwxr-x— 1 urbackup urbackup 0 Oct 16 03:26 backup_server_files.db-wal*
-rwxr-x— 1 urbackup urbackup 8192 Oct 16 03:26 backup_server_link_journal.db*
-rwxr-x— 1 urbackup urbackup 379072 Oct 16 03:26 backup_server_link_journal.db-wal*
-rwxr-x— 1 urbackup urbackup 16384 Oct 16 03:26 backup_server_links.db*
-rwxr-x— 1 urbackup urbackup 0 Oct 16 03:26 backup_server_links.db-wal*
-rwxr-x— 1 urbackup urbackup 61440 Oct 16 03:26 backup_server_settings.db*
-rwxr-x— 1 urbackup urbackup 86552 Oct 16 03:26 backup_server_settings.db-wal*
-rwxr-x— 1 urbackup urbackup 391 Oct 16 03:26 server_ident_ecdsa409k1.priv*
-rwxr-x— 1 urbackup urbackup 436 Oct 16 03:26 server_ident_ecdsa409k1.pub*
-rwxr-x— 1 urbackup urbackup 23 Oct 16 03:26 server_ident.key*
-rwxr-x— 1 urbackup urbackup 334 Oct 16 03:26 server_ident.priv*
-rwxr-x— 1 urbackup urbackup 442 Oct 16 03:26 server_ident.pub*

/media/BACKUP/urbackup/urbackup_tmp_files:
total 0

Any idea of what I’m doing wrong?